This invention relates generally to glass ceramic cooktop appliances and particularly to electronic power control systems for such appliances.
Commonly assigned U.S. Pat. No. 4,740,664 to Payne et al, which is hereby incorporated by reference, discloses a cooktop appliance equipped with heating units which radiate substantially in the infrared region (1-3 microns) in combination with a glass ceramic cooktop support surface which is substantially transparent to infrared radiation. Utensils placed on the cooktop surface are heated primarily by radiation directly from the heating unit rather than by conduction from the glass ceramic material. Though the glass ceramic is substantially transparent to the radiation, a portion of the energy radiating from the heating unit is absorbed by the glass ceramic, as is a portion of the energy reflected by the utensil being heated. Heat transfer from the glass ceramic is primarily by conduction to the utensil.
The power control system disclosed in the aforementioned Payne et al patent, using glass ceramic temperature information derived from a temperature sensor located directly over each heating unit controls the output power of each heating unit to protect the glass ceramic against overheating caused by abnormal load conditions such as operating the unit with no utensil present, use of badly warped utensils, or heating an empty utensil.
In that arrangement the temperature measurements are obtained by measuring the resistance of the bottom surface of the glass ceramic material above the heating units. The temperature information so obtained is sufficiently accurate for protecting the glass ceramic against overheating.
Since radiation is the primary heat transfer mechanism for utensils being heated on such cooktops, the system responds more quickly to changes in user selected power settings than the conventional cooktops relying on conduction heating. However, the thermal inertial of the glass ceramic material results in a slower response than that achievable with closed loop automatic surface unit systems which measure utensil temperature directly and control the output power of the heating unit to achieve and maintain the user selected utensil temperature. Inherent inaccuracies in this temperature measurement system due to the temperature gradient through the glass ceramic material, the temperature gradient from the top of the glass ceramic material to the bottom of the potentially warped pan, and other sources of error, render this temperature sensor arrangement incompatible with such closed loop systems. Locating a sensor to directly sense utensil temperature would add cost and complexity to the manufacturing process and by protruding above the cooktop would negate at least to some extent the appearance and cleanability advantages of the smooth cooktop surface. Hence, there exists a need for a control arrangement which provides a faster response to changes in power setting than that of typical open loop control systems while retaining the advantages in cost, cleanability and appearance of the smooth glass ceramic cooktop surface.
Therefore, it is a primary object of the present invention to provide an improved power control system for a glass ceramic cooktop appliance which reduces the time required for the system to reach steady state conditions in response to changes in the user selected power setting using a temperature sensor mounted to the bottom or inner surface of the glass ceramic cooktop support surface.